$_firstname=$_POST['firstname'];
            $_surname=$_POST['surname'];
            $_email=$_POST['email'];
            $_date=$_POST['date'];



         $db = mysqli_connect('localhost', 'root', '', 'database')
         or die('nie połączono');
     $query = "INSERT INTO users(firstname, surname, email, date)" .
             "VALUES($_firstname, $_surname, $_email, $_date)";

     mysqli_query($db, $query)
             or die('błąd');

     mysqli_close($db);